1. Technical Field of the Invention
The present invention relates to orthopedic stabilization devices used to limit the relative motion of at least two vertebral bodies for the relief of pain. These devices can be used to aid osteo-synthesis in combination with fusion devices, supplement other motion restoring devices such as disk implants or used solely to restrict the motion of vertebral bodies without other devices.
2. Description of the Related Art
In the field of spine surgery, there have been many attempts to relieve pain associated with spinal injury or illness. Traditionally surgeons have fused the vertebral bodies with a pedicle screw and rod construct or a fusion cage. In attempting to fuse the patient there is a long and painful recovery process. Most rod and screw constructs and fusion cage constructs are very rigid, not allowing transfer of stress into the fusion site that would otherwise aid in a quicker recovery. These approaches defy Wolfe's law stating: bone that is not stressed will degrade. As a corollary, where stress is allowed to transfer through the fusion site while the vertebral bodies are held in a limited range of motion, then fusion can occur much quicker aiding in patient recovery time.
Many are working to develop devices that allow relative motion, yet these have fallen short in minimizing shear forces between the vertebral bodies being stabilized. Another shortcoming is that relative motion has been forcibly channeled through a rather specific location or hinge point in the mechanical construct. The following discussion more particularly summarizes these efforts.
U.S. Pat. No. 5,092,866, which was reissued as U.S. Re. 36,221, discloses a pedicle screw system that is banded together with flexible ligaments. While the ligaments allow for relative motion, they do not appear to resist compression or shear loads, instead relying upon tension alone.
European Patent No. EP 06691091 A1/B1 and the DYNESYS®, a registered trademark of Zimmer GmbH, product brochure disclose a polycarbonate/urethane supporting element, compressed between two adjacent pedicle screws and passing over an elastic strap that acts as a flexible internal ligament. The flexible internal ligament is in the form of a nylon cord, which is pre-tensioned and fastened to the screw heads. This design provides flexural degrees of freedom, allows relative motion between the vertebral bodies, but does little to inhibit or prevent shearing between the vertebral bodies. While flexibility is desirable, the DYNESYS® ligament would appear to lack rigidity and rely on proper tensioning inter-operatively to gain its balance.
U.S. Pat. No. 6,267,764 discloses a pedicle screw and rod system wherein the rod is flexible in translation. A dampening ball is not separate from the rods and has cutouts to allow bending, with no ligament passing through the centers of the rods. While flexibility in translation can be helpful, the spine loads in several planes at the same time and the translation spoken of in this patent would appear to inadequately redistribute stresses through the fusion site. As a result motion is forcibly limited to one location, i.e., motion is constrained through a hinge point, which undesirably stresses the assembly construct.
U.S. Pat. No. 6,241,730 discloses a construction that lacks a ligament element, particularly a ligament extending through the center of rod members. There is a compressible dampening element. The disclosed design attempts to accomplish a multidirectional redistribution of force for aiding in quicker fusion rates, however its constructs were not designed for use in conjunction with a disk implant. The disclosed approach overly limits motion of the vertebral bodies to one location, i.e., forces motion unnaturally through a hinge point.
U.S. Pat. Nos. 6,293,949 and 6,761,719 disclose embodiments seeking to elastically constrain range of motion using a continuous super-elastic nitinol rod and pedicle screw system. Due to the super-elastic state of the rod, motion is always pushed-back to a neutral, pre-set position. This constrains force through the rod in a manner causing early fatigue failure. In order to provide the correct elasticity of the rod, its diameter must be so small that it cannot withstand the continuous loads. Further, the rod cannot be bent at the time of surgery to a preformed shape holding the vertebral bodies in a desired relative position while also limiting their relative motion.
Accordingly, there exists a need for assemblies and devices that effectively resist torsion as well as shear forces while providing flexible spine stabilization. More specifically, it would be desirable to provide kits with such assemblies and devices, which work with existing pedicle screw arrangements.
There is another need for flexible assemblies and devices having rigid members deformable to fit a patient's anatomical contours while maintaining flexibility of the orthopedic construct.
There is yet another need for assemblies and devices to stabilize vertebrae while providing multi-directional flexibility, without imparting elastic stresses to the bone.
There is a further need yet to provide a spine stabilization device that can allow natural flexion and extension motion while effectively restraining torsional and shear forces.
There is a further need to provide spine stabilization assemblies and devices manufactured from a shape memory material such as an alloy or other flexible polymer, which can withstand repeated loading of the spine without fatiguing yet still maintain its flexibility.